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Archived

Avalanche Forecast

Dec 20th, 2014–Dec 21st, 2014
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

Regions: Stevens Pass.

Unstable storm and wind slabs should persist and build near and above treeline Sunday.  Wet snow and rain below treeline should maintain areas of unstable wet snow. 

Detailed Forecast

Following a warm frontal passage Saturday night, strong moist westerly flow and generally mild temperatures should maintain moderate rain and snow through Sunday afternoon, tapering late Sunday. A likely convergence zone may form and affect the areas from Stevens to Snoqualmie Passes with locally heavier precipitation.

Rain should reach at least to near tree line elevations Saturday night and Sunday to cause a chance of natural avalanches or wet snow avalanches.

Above tree line, wet, heavy snow and very strong crest level winds should build further unstable storm and wind slabs. Dangerous avalanche conditions should persist Sunday with triggered avalanches remaining likely.

Snowpack Discussion

Although the strong storm that arrived early Saturday has deposited 10-15" as of Saturday evening, the overall snowpack remains limited below treeline. In general, there now is about 20-40" (50-110cm) from 3000-5000 feet along the west slopes. Periods of mild temperatures and several rain events have formed a very firm base layer below tree-line.  The overall still shallow snowdepths are maintaining many terrain anchors (trees, rocks, etc.), preventing large avalanches.  

Above 5000' to the top of our forecast range (7000-8000 ft)... snowdepths are still seasonally well below average, but are deep enough to provide a bed surface for new snow avalanches from the current storm snow.  Generally, there is 3-5 foot base above 5000', deepest in the North Cascades and at Mt. Rainier.  Expect a shallower snowpack in the Stevens-Snoqualmie zones and south of Mt. Rainier.

Professional NWAC observers, out Saturday 12-20 from the Mt Baker backcountry, to the central Cascades near Stevens Pass and just north on Jove Peak, all report that storm slabs are the primary avalanche problem with storm snow amounts ranging from 14-24" (35-60cm). In all areas, warming Saturday afternoon created an upside down new snow layering with denser snow overlying weaker colder snow below. This was causing easy new snow failures from a variety of stability tests. The recent snow overlies shallow firm old snow near the ground. Bonding of new snow to the old snow surface above ground varied, but was mostly weakly bonded.    

In the Mt. Baker area, new snowfall received Thursday and Thursday night in the Mt. Baker area led to a human triggered slab avalanche that released to the surface on a lee W-SW aspect near treeline where the new snow bonded poorly to the old crust.

Similar results were reported from the limited avalanche control at the Mt Baker ski area Saturday, with new storm snow reactive and releasing as soft slabs from ski cut and explosive means.   

50 cm storm slab on W-SW aspect at Mt. Baker ski area Friday, near tree-line. Photo by Garrett Elwood.

 

Avalanche Problems

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Wind Slabs

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Elevations: Below Treeline.

Likelihood: Likely

Expected Size: 1 - 1